a) Field of the Invention
In order to mount or assemble high-power diode lasers, it is known to solder laser bars to a heat sink having a thermal expansion coefficient which differs sharply from that of the laser bar by means of soft solder, e.g., an indium-containing solder or tin-lead solder. (1) (2) (see list of citations at end).
By laser bar is meant a strip of semiconductor material with a typical width of 10 mm which is divided, e.g., by V-grooves, as they are called, into single laser diodes which act optically and electrically like an arrangement of a plurality of parallel-connected individual laser diodes, but which are physically separated from one another. Accordingly, the expression "individual laser diodes" Einzellaserdioden! will be understood hereinafter as laser diodes which are physically separated from one another, while "the single laser diodes of a bar" einzelnen Laserdioden eines Barrens! refers to laser diodes which are physically connected with one another. Laser bars as a side with p-doping (p-side) where the active regions are located. The opposite side of the laser bar is designated as the n-side. By laser diodes are meant components with a so-called "broad area" or an array structure, as it is called, which emit laser light. By high-power diode laser is meant the entire arrangement comprising a laser bar, a heat sink, and the bonding of the n-side of the laser bar which is realized, e.g., by means of a soldered cover or by wire bonds.
b) Description of the Related Art
In the known mounting process, occurring as a result of the different thermal expansion coefficients of the laser bar and heat sink during the soldering process are compensated by the plastic flux of the soft solder. (1)
A disadvantage herein consists especially in the ageing of the solder location due to formation of intermetallic phases, whisker growth, and strong electromigration at the very high current densities which occur. (3) (7) With respect to long-term behavior, this leads to a deterioration of electrooptical characteristics and to a limiting of the life of such high-power diode lasers to a few thousand hours.
These disadvantages can be prevented through the use of a gold-tin solder which has low ductility at room temperature. However, as is reported in the literature, the use of such solder achieves only an insufficient compensation of mechanical stresses (4), which results in the destruction (5) of the semiconductor material or in accelerated degradation of the high-power diode laser (3).
In fact, when laser bars were soldered during tests to heat sinks having a substantially lower thermal expansion coefficient than the laser bar, microcracks were observed in the laser bar, which indicates extremely high mechanical stresses. Stress calculations confirmed this. Cracks in the region of the active zones of laser diodes destroy these laser diodes.
According to one teaching (8), the mechanical stability of the bond can be increased by filling the V-grooves with solder. (8) However, it is not possible to prevent mechanical stresses in this way.
In practice, the problem of crack formation can be avoided by mounting individual laser diodes. (9) It is also obvious to mount laser diode groups, by which is meant a physical unit of a plurality of single laser diodes, typically two to five individual laser diodes. However, when mounting individual laser diodes or laser diode groups, alignment problems occur due to the small geometric dimensions. Further, the optical power density is reduced owing to the required intermediate spaces between the individual laser diodes or laser diode groups. Moreover, this considerably complicates manufacture.